Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q21: Focus Session: Dopants and Defects in Semiconductors III |
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Sponsoring Units: DMP Chair: Michael Stavola, Lehigh University Room: 323 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q21.00001: High-Resolution Spectroscopy with a Free-Electron Laser: Vibrational Lifetimes of Hydrogen-related Defects in Silicon Invited Speaker: Gunter Luepke, Department of Applied Science, The College of William and Mary, Williamsburg, VA 23187 Vibrational lifetimes of hydrogen- and deuterium-related bending and stretching modes in crystalline silicon are measured by high-resolution infrared absorption spectroscopy and pump-probe transient bleaching technique using the Jefferson Lab. Free-Electron Laser. We find that the vibrational lifetimes of the bending modes follow a universal frequency-gap law, i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multi-phonon process. In contrast, the lifetimes of the stretching modes are found to be extremely dependent on the defect structure, ranging from 2 to 295 ps. Against conventional wisdom, we find that lifetimes of Si-D stretch modes typically are longer than for the corresponding Si-H modes. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems. The potential implications of the results on the physics of electronic device degradation are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q21.00002: First-Principles Calculation of Carrier Lifetimes in Semiconductors Vincenzo Lordi, Paul Erhart, Daniel Aberg We have developed first-principles methods based on density functional theory to calculate carrier lifetimes in semiconductors related to trapping on deep-level defects. Lifetimes are determined based on Schottky-Read-Hall theory, using recombination rates calculated from first principles for several possible, competing mechanisms: radiative recombination, phonon-assisted (nonradiative) recombination, and Auger recombination. The recombination rates are calculated within a fully first-principles framework with no empirical parameters. We have recently applied these methods to study the role of native and impurity defects in reducing carrier lifetimes in bulk single-crystal aluminum antimonide (AlSb) and cadmium telluride (CdTe), two promising materials for high-resolution room-temperature gamma radiation detection. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q21.00003: Fully \textit{ab initio} supercell corrections for charged defects Christoph Freysoldt, J\"{o}rg Neugebauer, Chris G. Van de Walle Charged point defects govern the carrier densities in semiconductors and are crucial for the performance of electronic devices. However, quantifying the thermodynamical, chemical, and electrical properties of such defects is a challenge to both experiment and theory. In \textit{ab-initio} calculations, the defect is usually modeled in a periodic supercell with a few dozen to a few hundred atoms. Unfortunately, this introduces artificial electrostatic interactions between charged defects. A number of correction schemes such as Makov-Payne corrections, potential alignment, scaling laws, or Coulomb truncation, are available in the literature, but they often fail to remove the supercell dependence completely. The assumptions behind these schemes are sometimes unclear and all schemes lack a stringent theoretical foundation. From a formal analysis within linear-response theory, we propose a new and simple scheme that combines the strengths of Makov-Payne corrections and potential alignment. Our scheme requires no empirical parameters or fitting procedures. Its reliability is demonstrated even in extreme cases. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q21.00004: Optimal Silicon for Photovoltaic Applications Georgy Samsonidze, Marvin L. Cohen, Steven G. Louie A small overlap of the silicon optical absorption spectrum with the solar emission spectrum limits the efficiency of silicon-based solar cells. We conduct a theoretical search for substitutionally doped silicon with the aim to maximize the spectral overlap. Different dopant species at various concentrations compatible with the existing silicon technology are examined in the virtual crystal approximation using the empirical pseudopotential method. The optimal doping configurations found are further investigated with a first-principles many-electron Green's function approach. The optical absorption spectrum of the doped silicon is calculated by solving the Bethe-Salpeter equation which includes excitonic effects. This work was supported by National Science Foundation Grant No. DMR07-05941, and by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NSF through TeraGrid resources at Indiana University and TACC. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q21.00005: Microprobe X-ray Absorption Spectroscopy of Chalcogen Doped Silicon Bonna Newman, Joe Sullivan, Mark Winkler, Meng-Ju Sher, Matthew Marcus, Matthew Smith, Silvija Gradecak, Eric Mazur, Tonio Buonassisi Doping Si with chalcogen atoms (S, Se, and Te) in excess of the solubility limit has been shown to result in optical absorption below the bandgap. This material, known as ``black silicon", is promising for infrared photon detectors and possibly photovoltaic devices. We report on the relationship between the chemical state of the dopant atoms and infrared absorption properties. A high concentration of 10$^{20}$ dopant atoms/cm$^{3}$ in the near-surface layer allows for extended X-ray absorption fine structure (EXAFS) investigations and determination of chemical state. We combine these results with absorption measurements and Auger spectroscopy to understand the correlations between optical and structural properties of chalcogen doped Si. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q21.00006: Hydrogen in multi-crystalline Si used for the fabrication of solar cells Chao Peng, Michael Stavola, W. Beall Fowler, Lode Carnel The multicrystalline Si materials that are used by industry to fabricate solar cells often contain a high concentration of carbon impurities. Furthermore, hydrogen is also commonly introduced during processing to improve solar-cell performance.[1,2] At present, the H- and C-related defect reactions that occur and what their effect might be remain poorly understood. We have performed a series of experiments in which IR spectroscopy has been used to study a family of defect complexes that are formed when H is trapped by substitutional carbon impurities in multi-crystalline Si. The structures, concentrations, and thermal stabilities of these defects have been investigated. 1. F. Duerinckx and J. Szlufcik, Sol. Energy Mater. Sol. Cells \textbf{72}, 231 (2002). 2. H. Dekkers, Dissertation, Catholic Univ. of Leuven, 2008. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q21.00007: Effect of copassivation of Cl and Cu on CdTe grain boundaries Lixin Zhang, Juarez L.F. Da Silva, Jingbo Li, Yanfa Yan, T.A. Gessert, Su-Huai Wei Grain boundaries (GBs) and dislocations are generally viewed as detrimental to device applications, because they usually contain a high density of deep defect levels that act as recombination centers for charge carriers. Surprisingly, two leading polycrystalline thin-film solar cells based on CuInSe2 (CIS) and CdTe have produced very high efficiencies of 20\% and 16.5\%, respectively, despite that these materials contain significant amounts of GBs. Using a first-principles method, we investigate the structural and electronic properties of GBs in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this work, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q21.00008: Influence of Crystalline Defects in GaN-InGaN Solar Cells Balakrishnam Jampana, Nikolai Faleev, Ian Ferguson, Robert Opila, Christiana Honsberg Crystalline defects originating from lattice-mismatch in epitaxial materials appear to be the dominant factor reducing high efficiency solar cell performance. In this paper we present an explanation of the observed structural and optical characteristics originating in lattice-mismatched III-nitride epitaxial materials. This model is based on creation, diffusion, accumulation and structural transformation of point defects to extended crystalline defects. In this work InGaN photovoltaic structures are grown by MOCVD on GaN templates with thicknesses in the 50 to 400nm range. The types and spatial distribution of crystalline defects are determined from XRD rocking curves and reciprocal space maps. The crystalline quality is observed to deteriorate with increasing thickness and growth rate. Wide band gap InGaN based solar cells require 150 to 400nm of active layer thicknesses and crystalline defects are observed in this thickness range degrading the solar cell performance. A physical model correlating the response of the solar cell to the type and spatial distribution of the defects will be presented. The work will aid improve the crystalline quality of InGaN for application as high efficiency solar cells. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q21.00009: Scanning Probe Spectroscopy of Individual Dopants in Silicon Morewell Gasseller, Matty Caymax, Roger Loo, Sven Rogge, Stuart Tessmer A key goal of semiconductor nanoelectronics is to develop devices based on manipulating the charge and spin of individual dopant atoms. Elucidating the quantum structure of these minute systems is a difficult technical challenge. Here we present capacitance-based scanned-probe measurements that both spatially-resolve individual subsurface boron dopants in silicon and detect spectroscopically single holes entering the B+ state of these atoms. We observe that, on average, acceptors with a closer nearest neighbor exhibit stronger binding. This finding is consistent with the interpretation of resonant tunneling measurements performed on a similar sample. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q21.00010: Vacancy-related defects and the E'$_{\delta }$ center in amorphous silicon dioxide Blair Tuttle, Sokrates Pantelides The microscopic identification of vacancy-related defects in silicon dioxide has been a major challenge. Particularly in amorphous silica, the role of vacancy clusters is still controversial. Experimental data have led to suggestions that the E'$_{\delta }$ center is a four-vacancy cluster instead of a single vacancy. Here we report density functional calculations of single vacancies and clusters of four vacancies in realistic models of amorphous silica. Results for single vacancies compare well to previous theory. A key result for four-vacancy clusters is that relaxations localize the unpaired electron preferentially on one Si atom, resulting in a strongly anisotropic electron-paramagnetic-resonance signal. Electrons at single vacancies have a more benign anisotropy which is more compatible with the observed isotropic signal. This work was supported by the Air Force Office of Scientific Research under a MURI grant (FA9550-05-1-0306) and by the US Navy. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q21.00011: Enhanced defect generation in gate oxides of p-channel MOS transistors in a moisture ambient Aritra Dasgupta, S.A. Francis, D.M. Fleetwood Transistors and ICs built in Sandia's 4/3 $\mu $m technology were exposed to moisture, irradiated, and annealed. The moisture exposures were performed using highly accelerated stress test (HAST) at 130$^{\circ}$C and 85\% relative humidity. Irradiation of n-channel transistors exposed to HAST followed by a long-term anneal resulted in some increase in interface-trap and oxide-trapped charge buildup. We observed enhanced post-irradiation defect generation of oxide trapped charge, interface traps and border traps in the gate oxides of p-channel MOS transistors that were exposed to humidity. This is characterized by enhanced voltage shifts due to oxide trapped charge and interface traps observed in the p-channel transistors. Low frequency noise measurements also showed enhanced low frequency noise power in the moisture exposed p-channel transistors. Our results indicate that there are enhanced precursor hole trap defects or oxygen vacancies present in the gate oxide of p-channel transistors as a result of presence of moisture or hydrogenous ambient. The smaller voltage shifts in the n-channel transistors may be related to the presence of phosphorus atoms in the gate oxides. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q21.00012: Carbon clusters as possible defects at the SiC-SiO$_{2}$ interface Yingdi Liu, Hongli Dang, Yang Liu, Ying Li, Matthew Chisholm, Trinity Biggerstaff, Gerd Duscher, Sanwu Wang High state densities in the band gap of the SiC-SiO$_{2}$ interface significantly reduce the channel mobilities in SiC-based high-temperature/high-power microelectronics. Investigations of the nature of the interface defects are thus of great importance. While several possible defects including very small carbon clusters with up to four carbon atoms have been identified by first-principles theory, larger carbon clusters as possible defects have attracted less attention. Here, we report first-principles quantum-mechanical calculations for two larger carbon clusters, the C$_{10}$ ring and the C$_{20}$ fullerence, at the SiC-SiO$_{2}$ interface. We find that both carbon clusters introduce significant states in the band gap. The states extend over the entire band gap with higher densities in the upper half of the gap, thus accounting for some of the interface trap densities observed experimentally. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q21.00013: First-principles study of local p(2$\times$2) structures on Si(100) surface Min-Kook Kim, Hyoung Joon Choi We study structural defects inducing local p(2$\times$2) structures in c(4$\times$2)-reconstructed Si(100) surface, using an \textit{ab-initio} pseudopotential density functional method. The local density approximation to the density functional theory is used and electronic wavefunctions are expanded with pseudo-atomic orbitals. The atomic structures of defects are optimized by minimizing the total energy. Our calculations show that the defects increase the total energy of the system but they are energetically stable with energy barrier. STM images for occupied and unoccupied states are simulated to investigate the surface electronic structures. Effects of electron doping and external electric field on the defects are also studied. This work was supported by the KRF (KRF-2007-314-C00075) and by the KOSEF Grant No. R01-2007-000- 20922-0. Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). [Preview Abstract] |
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